1. Field of the Invention
This invention is directed to an avalanche Light Emitting Diode which can be realized in silicon using a PN junction.
2. Prior Art
It is well known that an avalanching junction produces light. In particular, silicon PN junctions operated in the avalanche or breakdown mode produce visible light. However, when compared to the efficiency of a GaAs based LED, silicon PN junction diodes operated in the avalanche mode are quite inefficient. However, as pointed out in U.S. Pat. No. 5,438,210, even an inefficient silicon based LED can be useful in opto-coupler applications, especially if it can be integrated with standard silicon components. A monolithic opto-coupler is inherently lower in cost than a hybrid opto coupler which employs a separate GaAs based LED and a separate silicon detector.
Ideally, the integrated LED should be realized using standard silicon processing and be reliable. One silicon based LED, the porous silicon LED, requires processing steps different from normal silicon integrated circuit process steps and may have reliability issues. However, PN junction diodes operated in the avalanche mode can be made with standard silicon processing and are known to be reliable.
In one study it was shown that the light output from an avalanching silicon diode is very linear with current and has essentially a temperature coefficient of 0 when driven by a current source. Thus, an avalanche LED is amenable to linear applications without feedback being required. The study also determined that the quantum efficiency is 2e-5 which is high enough to make practical opto-couplers in applications were power is availible to amplify the light detector signal.
It is the objective of the this invention to show methods of constructing an avalanche PN junction diode which optimizes its light output and is stable over time. This is accomplished by judicious placement of doping implants relative to electrodes, electrode construction, and construction of light reflecting mirrors. Two types of diodes are considered including a vertical junction avalanche diode and an edge avalanche diode. In the case of the edge avalanche diode, the heavy implant region corresponding to the avalanche zone must be placed somewhat below the surface so that hot carrier injection into the passivating thick field oxide is avoided. A second version of the edge avalanche LED is also shown in which an SOI substrate is used.
A. Lacaita, F. Zappa, S. Bigliardi, and M. Manfredi, xe2x80x9cOn the Bremsstrahlung Origin of Hot-Carrier-Induced Photons in Silicon Devicesxe2x80x9d, IEEE Trans. Electron Devices, vol. ED-40, p.577, 1993.